Vibration isolation systems suppress vibrations acting on a structure or payload. Passive isolators, such as dampers and absorbers, are mechanical solutions that, while effective, have difficulty suppressing vibrations at low frequencies. Active isolation systems have been developed to suppress vibrations acting on a structure or payload at low frequencies. Generally, active isolation systems measure vibrations at specific locations on a platform bearing a payload and dynamically apply cancellation forces in an equal and opposite direction to suppress the effect of the vibration. The vibrations sensed by these dynamic systems are provided to a processor which activates an actuator that applies the cancellation forces.
Two typical applications for active isolation systems include environments with a high level of ground noise, such that the ground noise affects the operation of an the equipment, and instruments that are very sensitive to small payload vibrations, such as atomic force and scanning tunneling microscopes.
As mentioned above, active isolation systems have the advantage of suppressing vibrations at low frequencies, such as below 2-3 Hz, but tend to be complex. An example of such a system is disclosed in U.S. Pat. No. 5,660,255. To suppress and isolate a payload from vibrations originating from the ground, an actuator is used to cancel the dynamic forces acting on an intermediate mass. In such a system, the actuator is expected to bear all of the weight (i.e., the static force) applied to the top of the actuator, which includes the weight of the intermediate mass, payload platform, passive support elements between the intermediate mass and the platform, as well as the weight of the payload itself. In addition to bearing the static forces, the actuator must generate sufficient forces in response to the movement of the intermediate mass to cancel any dynamic forces acting on the intermediate mass, so that vibrations can be suppressed to an acceptable level.
To provide the dual functional demands of supporting static weight and providing forces needed to isolate the payload from vibration, the actuator generally employed is a piezoelectric stack. This type of actuator can be quite expensive; therefore, to reduce the strain and wear on the actuator, some systems combine the actuator with other passive support elements, such as an offload or support spring, to bear a portion of the static weight. However, as disclosed in U.S. Patent Application Publication No. 2010/0030384, it is generally accepted that the extent to which an offload spring may reduce the burden on the actuator by bearing a portion of the static weight is limited because of the risk that the offload spring would compromise the efficiency of the vibration isolation system. In particular, because the support springs extend from the ground, vibration at frequencies below the resonance frequency of the springs may transfer from the ground through the springs resulting in compromised performance of the active dampening portion of the vibration isolation system.